Human monocytes separated from peripheral blood by Ficoll-Hypaque and by adherence to serum-coated dishes show a bimodal volume distribution measured with a fluorescence-activated cell sorter. In the first peak of size distribution histogram of living mononuclear cells, lymphocytes and small monocytes were characterized by latex phagocytosis and non-specific esterase staining, whereas in the second peak the large monocytes dominated. The percentage of esterase stained small monocytes was lower than that of the large ones. Parallel to these data, the rate of the FDA hydrolysis of the small monocytes was lower than that of the large ones. The majority of the large monocytes reacted with sensitized sheep red blood cells (sSRBC) while only the minority of the small monocytes bound sSRBC. Scatchard plots on the binding of fluorescein isothiocyanate (FITC)-labelled human monoclonal IgG1 to the two subpopulations indicated similar association constants. K = 1 . 2 +/- 0 . 3 X 10(5) M-1. The number of Fc receptors was significantly different for the small (3 . 3 +/- 0 . 6 X 10(5)) and the large monocytes (10 +/- 1 X 10(5)).